Liquid crystal display devices are thin and light-weighted and require small power consumption. Accordingly, liquid crystal display devices are widely used in electric devices such as mobile phones and personal digital assistants (PDA). In the liquid crystal display devices used in these electric devices, a lighting device, which is generally referred to as a backlight, is provided.
FIG. 1 is a schematic view depicting one example of a conventional liquid crystal display device (transmission-type liquid crystal display device). As depicted in FIG. 1, the transmission-type liquid crystal display device includes a liquid crystal panel 10 and a backlight 20 arranged on the back surface side of the liquid crystal panel 10.
The liquid crystal panel 10 is formed by enclosing a liquid crystal 12 between two transparent substrates 11a and 11b. In addition, a polarizer 13a is arranged on the front surface side (upper side in FIG. 1) of the liquid crystal panel 10 and a polarizer 13b is arranged on the back surface side thereof (lower side in FIG. 1).
The backlight 20 includes a light emitting diode (LED) 21 serving as a light source, a light guide plate 22, a reflection sheet (mirror or white sheet) 23, and a prism sheet 24. The LED 21 is arranged on one end surface (light-entering surface) side of the light guide plate 22. In general, three or four LEDs 21 are used in a 2-inch liquid crystal panel.
The light guide plate 22 is formed of a transparent resin so that the cross-section thereof is a wedge shape as depicted in FIG. 1. The reflection sheet 23 is arranged on the back surface side of this light guide plate 22, and the prism sheet 24 is arranged on the front surface side (liquid crystal panel 10 side). In the following description, the back surface and front surface of the light guide plate 22 are respectively referred to as a reflection surface and a light-outgoing surface.
In the liquid crystal display device configured as described above, light emitted from the LED 21 enters the light guide plate 22 from an end surface (light-entering surface) of the light guide plate 22. The incident light is repeatedly reflected between the two surfaces (reflection surface and light-outgoing surface) facing to each other in the thickness direction of the light guide plate 22 and finally is outputted from the light-outgoing surface to the outside of the light guide plate 22. The prism sheet 24 refracts the light outputted from the light guide plate 22 in a direction substantially perpendicular to the light-outgoing surface of the light guide plate 22.
In general, a pixel electrode is formed for each pixel on one of the two transparent substrates 11a and 11b configuring the liquid crystal panel 10, while a common electrode facing to the corresponding pixel electrode and a color filter are formed on the other substrate. When the light emitted from the backlight 20 passes through the polarizer 13b arranged on the back surface side of the liquid crystal panel 10, the light becomes linearly polarized light. When a voltage is applied between the pixel electrode and the common electrode, a phase of the light passing through the liquid crystal 12 changes according to the voltage. Thereby, an amount of the light transmitting through the polarizer 13a on the front surface side can be controlled. By controlling the transmission amount of the light for each pixel, a desired image or character can be displayed on the liquid crystal display device.
In the transmission-type liquid crystal display device, it is preferable that the entire surface of the liquid crystal panel 10 is uniformly irradiated with the light emitted from the backlight 20. For this reason, in some transmission-type liquid crystal display devices, fine unevenness is provided on a reflection surface and a light-outgoing surface of the light guide plate 22 to diffuse light more uniformly.
As described above, a general transmission-type liquid crystal display device requires a lighting device (backlight). In contrast, a reflection-type liquid crystal display device does not need a lighting device because it performs display by use of reflection of natural light or illumination light. However, even some of such reflection-type liquid crystal display devices include a lighting device, which is referred to as a front light, on a front surface side of a liquid crystal panel, because a screen becomes hardly visible when an ambient condition becomes dark. Similarly to the backlight, the front light is also formed of a light guide plate and a light source arranged around an end surface of the light guide plate.
In the meantime, as depicted in FIG. 2, in a conventional lighting device for a liquid crystal display device, there may occur uneven brightness in which streak portions diagonally extending from the end surface on the side on which a light source is arranged become brighter than the other portions. This uneven brightness is caused by the following reasons.
To be more specific, as depicted in a perspective view in FIG. 3A, multiple cylindrical lenses 25, each extending in the length direction (Y-axis direction depicted in FIG. 3A) of the light guide plate 22, are generally arranged on a reflection surface of the light guide plate 22. These cylindrical lenses 25 are arranged so that light emitted from a light source would be diffused in the width direction (X-axis direction depicted in FIG. 3A) of the light guide plate 22 so as to cause brightness to be uniform. However, as depicted in the cross-sectional view in FIG. 3B, the light incident from LED 21 into the light guide plate 22 is reflected in the vertical direction (thickness direction, or the Z-axis direction depicted in FIG. 3B) at boundary portions (pointed portions) between the cylindrical lenses 25 and is outputted from the light-outgoing surface of the light guide plate 22 to the outside. This is how the uneven brightness is caused.
To avoid such problem, as depicted in FIG. 4, Japanese Laid-open Patent Publication No. 2004-6326 proposes that the surfaces of the cylindrical lenses 25, formed on a reflection surface side of a light guide plate 22, in a portion close to light sources (hatched portion in FIG. 4), are roughened, and that the resultant rough surfaces of the cylindrical lenses 25 are used to diffusely reflect light. In addition, Japanese Laid-open Patent Publication No. 2005-71610 discloses a light guide plate 31 in which cylindrical lenses 32 (protruded portions extending in the vertical direction to a light-entering surface) are provided on a light-outgoing surface side thereof as depicted in the cross-sectional view in FIG. 5. In this light guide plate 31, each boundary portion between the cylindrical lenses 32 is formed by a curved surface.
However, the present inventors consider that the methods disclosed in Japanese Laid-open Patent Publications Nos. 2004-6326 and 2005-71610 have the following problems. That is, the method disclosed in Japanese Laid-open Patent Publication No. 2004-6326, requires blasting as the surface roughening process, for example. The blasting is a process of forming unevenness by blasting sand (abrasive grains) onto a mold used in forming a light guide plate. In this case, it is required that a step of performing blasting on a mold, a step of manufacturing a light guide plate by using the blasted mold, and a step of evaluating optical characteristics of the manufactured light guide plate be repeated to manufacture a mold that can produce a light guide plate with a desired characteristic. However, the blasting is poor in reproduction of the uneven pattern. Thus, a longer time is required until a mold with a desired quality is obtained, which results in increasing the manufacturing cost.
In the light guide plate disclosed in Japanese Laid-open Patent Publication No. 2005-71610, the cylindrical lenses are provided on the light-outgoing side. Accordingly, there is a problem of deteriorating the brightness distribution characteristic because the light incident from the light source into the light guide plate is outputted to the liquid crystal panel side without being sufficiently diffused by the light guide plate.